Passat TDI 170 Cruise Control Fault

[passat170]

I bought a new Passat SEL TDI 170 DSG on 1st Sept 06.
From new the car has had a fault with the cruise control.
The button on the end of the cruise control stalk may be depressed to reduce the speed, when cruise control is operative, in 1mph increments. The stalk may also be pulled towards the driver to increase the speed in 1mph increments. The stalk may also be moved up or down to adjust in 5mph increments.
The fault occurs when decreasing the speed by pressing the button on the end of the stalk - if this button is repeatedly pressed (as you need to do if descending on a vehicle in front) the speed reduces by 1mph on some occasions and increases by 1, 2, 3, 4 or even 5mph on some occasions. This is clearly an unacceptable fault!
The VW dealer could not diagnose the cause of the fault.
VW Technical asked the dealer to try another Passat - they did so and told me that the same problem was evident on this other Passat.
So it would appear to be a common fault and I suspect this to be a problem with the ECU software design.
Note that one would not necessarily expect either the on-board diagnostics or the dealer's diagnostic equipment to find a design fault in the ECU software.
The VW dealer spoke to VW Technical and fobbed me off with an excuse that I didn't accept.
VW similarly tried to fob me off and I have not accepted their excuse - I asked for this to be escalated and await their further response.
QUESTIONS:
Does anybody own a Passat with a similar fault on the cruise control?
Can anybody share any further information on this?

[Flash2]

I never knew you could increase/decrease the speed in 1 MPH increments via the control stalk. When the fault occurs, does it display an increase in speed on the instrument cluster display? (on the multi function display next to the cruise control symbol) I'll try and check this out for you and I'll post my findings.

Regards - Jim.

[passat170]

Thanks for reply Jim!
Yes when the fault occurs the indicated speed on the digital display increases by 1, 2, 3, 4 or 5mph (and the car does go faster).
If you pause a while before pressing the tap down button again it seems to work fine. But if you press the button repeatedly this is when the fault occurs.
I have never experienced this on any of my 3 previous Golf TDIs or any other car I have driven with cruise control.
I'm eagerly awaiting further news from you!
Jonathan

[BenR]

This is news to me as well. I thought you could only + or - by about 5 mph by raising or lowering the stalk. I will try this on the way home tonight, rain and traffic permitting!

[BenR]

Tried it this morning - raising or lowering the stalk gives you a 5 mph increase or decrease in speed. Pushing the button on the end gives you a 1 mph decrease in speed. Useful for fine adjustments.

[Flash2]

I had a chance to check this out today too. My vehicle seemed to do the same as Jonathan's. I set the cruise to hold 35MPH then I pressed the SET button. The speed decreased in 1MPH steps down to 32 then it jumped back up to 34. Pulling the lever towards me (RESUME) increased the speed in 1MPH steps, but it never jumped to a lower speed. I also connected the car to my diagnostic machine and read the measured values from the switch but the steering control unit seemed to be reading the switch OK.

The vehicle I used didn't have the owner's manual in it, so I couldn't check to see if this is an actual feature of the system. Have any of you two checked the book to see if it says anything about this?

Regards - Jim

[passat170]

Yes Jim,
The manual says the following:
"Briefly press button SET to decrease the desired speed by approx 1 km/h. Press button SET repeatedly to reduce the speed by several km/h."
So the functionality is not working as intended.
The dealer I took mine too tried another 170 TDI at VW Technical's request and they told me the same fault was evident so I believe it's a common fault on the 170 TDI's.
Can you report your similar fault to VW (I guess your Passat is on warranty)? It will help - if others report the fault they are more likely to correct the error in the software.
There's probably little point in the dealer investigating yours further as the on-board diagnostics and dealer's diagnostic equipment will not find problems arising from an ECU software design error (which I suspect to be the case).
Cheers,
Jonathan

(the SET button being the button on the end of the stalk)

[passat170]

Ben,
Please try repeatedly pressing the button on the end of the stalk and see if occasionally the speed increases (when it should only decrease).
Thanks,
Jonathan

[BenR]

Okay. Back soon.

[carl s]

I've only ever managed to get the speed to increase or decrease in 5mph increments. I'll have to try using the set button, although not sure how it knows to decrease or increase by 1mph I take it's just arbitrarily decreasing then ?